1. Field of the Invention
The present invention relates to an image transfer unit of an image formation apparatus such as an electrophotographic copying apparatus or a printer.
2. Discussion of the Background
Conventionally, in an image transfer unit of an image formation apparatus, toner particles deposited on a latent electrostatic image formed on an image bearing member are charged to a predetermined polarity, and an electric field is applied across the image bearing member and an image transfer roller. When a transfer sheet is allowed to pass through a contact portion between the image bearing member and the image transfer roller, the toner image on the image bearing member is transferred to the transfer sheet.
In a conventional image transfer unit of an image formation apparatus employing a belt-shaped photoconductor as an image bearing member, as shown in FIG. 12, a supporting and tension-application roller 1 and a supporting and tension-application roller 2, located in parallel with each other, supports and provides tension to a belt-shaped photoconductor 3, and an image transfer roller 4 which is situated so as to be parallel with the axial direction of the supporting and tension-application rollers 1 and 2 is in contact with the belt-shaped photoconductor 3.
In the above-mentioned image transfer unit, since the belt-shaped photoconductor 3 is made bend by a slight contact with the image transfer roller 4, the friction between the image transfer roller 4 and the belt-shaped photoconductor 3 becomes small, so that image transfer cannot be satisfactorily carried out because of slip of an image transfer sheet 6.
In order to eliminate the above-mentioned inconvenience, for example, Japanese Laid-Open Utility Model Application 60-44060 discloses an image transfer unit in which an image transfer roller is brought into pressure contact with a belt-shaped photoconductor in such a configuration that the belt-shaped photoconductor is made slightly bend along the circumference of one of supporting and tension-application rollers. Owing to such a configuration, the friction between the image transfer roller and the belt-shaped photoconductor can be increased.
The above-mentioned conventional image transfer unit comprising the belt-shaped photoconductor serving as the image bearing member, however, tends to produce an improper image when used with a transfer sheet with a high stiffness such as a sheet of thick paper or a film for use with an OHP. This is because the stiffness of the transfer sheet 6 surpasses the contact force between the belt-shaped photoconductor 3 and the image transfer roller 4, and the belt-shaped photoconductor 3 is therefore partially pressed to produce a gap 11 between the image transfer roller 4 and the transfer sheet 6 or between the image transfer sheet 6 and the belt-shaped photoconductor 3 while the transfer sheet 6 is passing between the belt-shaped photoconductor 3 and the image transfer roller 4. Namely, the transfer sheet 6 is not exactly sandwiched between the image transfer roller 4 and the belt-shaped photoconductor 3, so that the toner image formed on the belt-shaped photoconductor 3 corresponding to the gap 11 is not transferred to the transfer sheet 6.
When the pressure of the image transfer roller 4 applied to the belt-shaped photoconductor 3 is increased not to form the gap 11, there are problems in that the image transfer roller 4 gives the belt-shaped photoconductor 3 a deep thrust, and at the same time, the image transfer roller 4 receives the repulsion force caused by the tension of the belt-shaped photoconductor 3. As a result, the image transfer roller 4 cannot be settled at a definite position.
Moreover, it is difficult to drive a plurality of rollers such as the supporting and tension-application rollers and the image transfer roller to rotate at the same speed by using the respective driving systems or by using an engagement system for transmitting the driving force to the respective rollers. For instance, when a plurality of rollers is driven to rotate by the engagement system such as a series of gears, the number of revolutions of each roller can be adjusted to the desired value but it is difficult to substantially control the moving speed of the surface of each roller to a desired value because of dispersion in the mechanical accuracy with respect to the diameter, and deflection of each roller. In order to allow a plurality of rollers to rotate at the same speed, when these rollers are brought into direct contact with each other to move by dragging, the manufacturing cost of the image transfer unit can be retrenched, and the rollers can be driven to rotate almost at the same speed.
In the relation between the belt-shaped photoconductor 3 and the image transfer roller 4, the resistance is produced by the moment of inertia, and the bearing friction thereof. When this resistance becomes larger than the friction at a contact portion between the belt-shaped photoconductor 3 and the image transfer roller 4, slip occurs therebetween, and dragging of both cannot be achieved properly. In particular, it is important to drive the belt-shaped photoconductor 3 and the image transfer roller 4 to rotate at the same speed in the sense that the toner image on the belt-shaped photoconductor 3 can be accurately transferred to the transfer sheet 6 without the occurrence of enlargement and reduction of the transferred image and slip of the transfer sheet 6.
When a drum-shaped image bearing member is employed in an image transfer unit of an image formation apparatus, the drum-shaped image bearing member comprises a metallic drum, for example, made of aluminum, and a photoconductive material layer formed thereon.
In the conventional image transfer unit comprising the above-mentioned drum-shaped image bearing member, the image transfer roller is also charged in the course of the image transfer operation, and attracting force works between the drum-shaped image bearing member and the image transfer roller. Therefore, this involves the problem that the transfer sheet, after passing through the contact portion between the image bearing member and the image transfer roller, still adheres to the surface of the drum-shaped image bearing member without separating therefrom, since the adhesion between the transfer sheet and the image bearing member surpasses the stiffness of the transfer sheet. When the charge quantity of the transfer sheet is increased, the transfer sheet is thin or has a low stiffness, or the curvature radius of the drum-shaped image bearing member is large, this problem frequently occurs.
In addition to the above, the image transfer roller is conventionally brought into pressure contact with the image bearing member only by use of pressure-application means such as a pressure-application spring. However, the pressure applied by the pressure-application spring varies depending on the kind of pressure-application spring, and the pressure-application spring changes with time, whereby the contacting state of the image transfer roller with the image bearing member easily becomes unstable. Thus, the image transfer operation cannot always be performed under appropriate conditions.